FLUID INCLUSION STUDIES OF ZONING IN THE DACHANG TIN-POLYMETALLIC ORE FIELD, PEOPLES-REPUBLIC-OF-CHINA

The Dachang tin-polymetallic ore field, Guangxi, People's Republic of China, is one of the largest in the world and contains the Changpo, Bali, and Longtaoshan tin-base metal the Dalfulou black shale-hosted cassiterite-sulfide deposit, and the Lamo Cu-Pb-Zn proximal skarn deposit. These are hosted by Devonian carbonate-rich sediments metamorphosed to hornblende hornfels facies near the underlying Yanshanian Longxianggei granite (91 Ma). Plutonic intrusion and relevant mineralization are controlled by the north-northwest-trending Longxianggei and Changpo anticlines. The conduit for distal skarn solution was a reverse fault (F1). Tin mineralization at Changpo is genetically related to the S-type Longxianggei granite with proximal skarns precipitated near its contact and distal ores 300 to 500 m vertically away from the contact. Hydrothermal alteration is grossly zoned outward from the granite contact as follows: (a) proximal ore skarn zone (garnet and pyroxene), (b) tourmaline-silicified zone, (c) sericite (+/- cassiterite) zone, and (d) carbonate(-sulfosalt) vein zone. Zones b to d do not occur above the Lamo deposit whereas zone a is missing directly below the large Changpo deposit but occurs lateral to it. Nearest to the granite outcrop, the Lamo Cu-Pb-Zn proximal skarn is developed with a temporal sequence of stages: (A) garnet-vesuvianite-wollastonite +/- diopside, (B) Ca amphibole-epidote-fluorite-quartz, (C) biotite-sericite-sulfides, and (D) sulfides-carbonate-chlorite. Distal skarns have the following temporally and spatially distributed stages: (1) tourmaline-quartz-cassiterite, (2) arsenopyrite-cassiterite (+/- other sulfides), and (3) sulfosalts-carbonate (most distal). There is a zonal pattern of both temperatures and ore fluid compositions deduced from fluid inclusions as follows: proximal skarn zone: 450-degrees to 600-degrees-C, CO2/CH4 < 0.01, total salinity >35 wt percent NaCl equiv; tourmaline-silica zone: 350-degrees to 450-degree-C, CO2/CH4 = 0.10 to 10.00, salinity = 12 to 30 wt percent NaCl equiv; sericite zone: 250-degrees to 350-degrees-C, CO2/CH4 >10, salinity <10 wt percent NaCl equiv; and carbonate-sulfosalt zone, <250-degrees-C, CO2 dominated, no CH4, salinity = 6 to 8 wt percent NaCl equiv. Daughter crystals include antarcticite (CaCl2.6H2O), halite, sylvite, carbonate, MgCl2 . 6H2O, and fluorite. Thermodynamic calculations based on fluid inclusion composition and measured temperature show that f(O2) values of the on-forming stages were very low, well below the FMQ buffer and increased from early to later stages. Tin precipitation was affected by a low oxygen fugacity of ore fluids partly resulting from the occurrence of black shale, and a decrease in temperature.